You are answering a USMLE-style medical question. Question: A stillborn infant is delivered at 38 weeks' gestation to a 32-year-old woman. The mother had no prenatal care. Examination of the stillborn shows a small pelvis, shallow intergluteal cleft, and club feet. An x-ray shows the absence of the sacrum and lumbar vertebrae. Which of the following is the strongest predisposing factor for this patient's condition? Options: A) Maternal oligohydramnios B) Intrauterine alcohol exposure C) Maternal diabetes D) Chromosome 18 trisomy Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.